On Thu, 14 Mar 2019 at 23:06, David Rowley <david.row...@2ndquadrant.com> wrote: > On Thu, 14 Mar 2019 at 21:12, Edmund Horner <ejr...@gmail.com> wrote: > > I'm not sure how an unreasonable underestimation would occur here. If > > you have a table bloated to say 10x its minimal size, the estimator > > still assumes an even distribution of tuples (I don't think we can do > > much better than that). So the selectivity of "ctid >= <last ctid > > that would exist without bloat>" is still going to be 0.9. > > Okay, think you're right there. I guess the only risk there is just > varying tuple density per page, and that seems no greater risk than we > have with the existing stats.
Yeah that is a risk, and will probably come up in practice. But at least we're not just picking a hardcoded selectivity any more. > Just looking again, I think the block of code starting: > > + if (density > 0.0) > > needs a comment to mention what it's doing. Perhaps: > > + /* > + * Using the average tuples per page, calculate how far into > + * the page the itemptr is likely to be and adjust block > + * accordingly. > + */ > + if (density > 0.0) > > Or some better choice of words. With that done, I think 0001 is good to go. Ok, I'll look at it and hopefully get a new patch up soon. Edmund
